In recent years, a power semiconductor device using a silicon carbide semiconductor device, that is, a semiconductor device having a silicon carbide (SiC) layer has been put into practical use, and a study for improving its reliability has been conducted. Since SiC itself has high dielectric breakdown strength, a dielectric breakdown in the silicon carbide semiconductor device easily occurs not in the SiC layer but in an insulating film provided thereon. Hence, it is important to prevent degradation of the insulating film to secure the reliability. Particularly, in a silicon carbide semiconductor device having an insulated gate structure, such as a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and IGBT (Insulated Gate Bipolar Transistor), it is desirable to prevent a dielectric breakdown of a gate insulating film.
An SiC-MOSFET or an SiC-IGBT serving as a practical power semiconductor device normally has p-type well regions facing each other with an n-type drift layer in between. A part of the drift layer sandwiched between the well regions is also referred to as a JFET (Junction-Field-Effect-Transistor) region. When the power semiconductor device is in an off state, a high electric field is applied to a gate insulating film located on the JFET region. Because of this, a dielectric breakdown of the insulating film especially easily occurs in a gate electrode on the JFET region, and various techniques for preventing this have been proposed (for example, the following Patent Documents 1 to 3).
Patent Documents 1 to 3 disclose a configuration in which a p− region is formed in an upper center of a JFET region in a MOSFET (a part where a threading dislocation exists in Patent Document 3). According to this configuration, when the MOSFET is in an off state, depletion of an upper part of the JFET region is promoted, thereby suppressing electric field strength applied to a gate insulating film at the upper part of the JFET region. Therefore, a breakdown of the gate insulating film when high voltage is applied to a semiconductor device can be prevented.